Static buildup on preprinted sheet material is an historic and troublesome problem for the printer. Static charges tend to build up when the sheet or web material is transported through a printing press. The resulting static buildup can cause web/sheet guidance problems. A second problem involves particulate contamination on the surface of the preprinted sheet. Particulate contamination is strongly attracted to a statically charged sheet or web and often degrades print quality.
Printing press manufacturers have attempted to deal with the static charge buildup problem by using various types of static elimination devices which are wrapped around printing rolls or festooned around a printing press. These devices conduct static charges to a ground by contacting or dragging against the surface of the sheet or web material as it passes through the press.
For example, in U.S. Pat. No. 3,235,772 to Gurin, a printing blanket is provided for conducting electrostatic charges away from a sheet to an underlying metal roll mounting the blanket. The printing blanket may have an elastomeric outer layer with conductive pigment embedded therein in contact with conductive intermediary and/or backing layers wrapped on the printing cylinder. In U.S. Pat. No. 3,867,027 to Weigl, drum rollers made of an electrically conductive mat of metallized fibers are provided for removing static charges from continuously transported sheet material. In U.S. Pat. No. 5,508,879 to Kitamura, a charge removal brush for a photocopier is constructed with long conductive filaments planted in a conductive cloth wrapped around a metal shaft.
In U.S. Pat. No. 3,635,158 to Budinger, a printing roller is provided with a high density felt of polyester fibers adhered to an underlying core for removing debris and particles from transported sheets. The felted layer can be formed on a heat-shrinkable tube of PET fibers that is slipped over the roller core and shrunk down to securely grip the roller core. The felted layer is used to remove particles from the transported sheets, but is not used for static charge removal.
The prior art thus shows the general concept of using a printing roll blanket or brush having an outer conductive layer or fibers for removal of static charges to a conductive core, or a felted layer as a roll cover for particulate removal. The prior art devices are effective under limited conditions but do not solve the problem entirely. Trials have shown that in some circumstances the use of a cover to remove particulates from the sheets can exacerbate static problems and cause particulate contamination to be attracted to the sheets later in the process. It is especially desirable to provide a transfer roll cover that is convenient to install and replace on a transfer roll and that can both dissipate static charges effectively and remove particulate contaminants from printed sheet or web material.